Add minimal mineral N cycle

[dlebauer]

Starting with eq 15 specified in the model structure documentation:

$$ \frac{dN_\text{min}}{{dt}} = F^N_\text{litter,min} + F^N_\text{soil,min} + F^N_\text{fix} + F^N_\text{fert,min} - F^N_\mathrm{vol} - F^N_\text{leach} - F^N_\text{uptake} $$

The first step is to implement the core mineral N cycle, i.e.

$$ \frac{dN_\text{min}}{{dt}} = F^N_\text{fert,min} - F^N_\mathrm{vol} - F^N_\text{leach} $$

Each pull request should include

• updated documentation
• tests for new feature
• updated changelog